Instruments which rely upon accurate control of a fluid stream are commonly employed in a wide variety of applications, such as chromatography, chemical analysis, clinical assay, sample purification, and industrial processing. Such instruments typically function through devices that operate by initiating, maintaining, halting, or reversing a fluid stream through the instrument. Such instruments can employ one or more fluid streams in respective flow paths. The various flow paths are provided in a flow system that combines flow-through components, such as channels, sorbent columns, and connective tubing, with terminal components, such as needles, pumps, and drains. Selected flow paths are frequently employed to, for example, isolate a component from the flow system, attach a component into the flow system, or rearrange the order of certain components in the flow system. For many systems, an extensive and complex array of channels, tubing, fittings, and the like are employed to provide the many flow paths that are necessary for optimum operation.
Certain characteristics of a fluid stream may be expressed as a function of both time and space. Both density and velocity of a fluid stream may be dependent on the time or the location of plural fluid elements that comprise a fluid stream within a channel. Thus it is generally useful in fluid mechanics to choose a convenient origin of coordinates and to study only the velocity field of the fluid stream as a function of time is known as the eulerian formulation of motion. The eulerian velocity vector field can be defined in the following Cartesian form: EQU V(x, y, z, t)=iu(x, y, z, t)+jv(x, y, z, t)+kw(x, y, z, t)
wherein t is time, u, v, w are scalar variables and x, y, and z are coordinates in the Cartesian coordinate system.
In what is known as the steady flow of a fluid, the velocity and density of the fluid in a channel are observed to be independent of time, although each characteristic may vary from point to point from within the fluid stream. In an unsteady flow, the velocity and/or density are variable with time. An unsteady flow often develops in a fluid stream that encounters an abrupt transition, such as a step, corner, restriction, or other discontinuity in the channel that bears the fluid stream.
According to boundary-layer theory, the state of a fluid stream in a channel is most likely to be affected by at least two length dimensions. Viscous effects at the boundary walls of the channel are found to retard the fluid. A distance into the fluid stream, known as the boundary layer, represents the decelerating effects of wall friction. As the length of the flow path increases, the thickness of the boundary layer also increases, and with sufficient length of the flow path, the boundary layer will fill the channel completely. One application of boundary-layer theory is the determination of the streamwise component of a fluid stream 100 entering a channel 102, as illustrated in FIG. 1. The velocity profile in a well-rounded entrance 104 is nearly uniform. Then, at x=0, a shear layer 106 begins. Due to continuity requirements, a retardation of flow near the channel wall 112 causes the so-called potential core 108 to accelerate and the boundary-layer growth 110 is thinned. At some finite distance downstream, the shear layers meet, and the duct is filled with boundary layer. Shortly thereafter, at x=x.sub.L, the flow is within approximately 1 percent of its final unaccelerated flow profile 114, e.g., the Poiseuille parabola. The flow thereafter is said to be fully developed and follows Poiseuille's law. The region 0&lt;x&lt;x.sub.L is known as the entrance section and x.sub.L is known as the entrance length.
The velocity of a fluid stream is denoted by a figure of merit known as the Reynolds number, or Re. At a low velocity (i.e., a low Reynolds number), the fluid stream tends to continue its flow in an orderly, laminar, manner. When the velocity is increased, there is a transition point (the critical Reynolds number) at which a fluid stream will not maintain laminar flow and instead becomes unsteady. For example, critical Reynolds numbers of 25 are found in fluid flow that encounters a sharp discontinuity or transition.
It is desirable to maintain a steady or laminar flow of a fluid stream in certain flow systems. The provision of a steady fluid stream in a pneumatic assembly becomes exceedingly problematic when the assembly is operated at a flow rate that exceeds the critical Reynolds number, or when the flow system is reduced in size (which tends to lower the critical Reynolds number). In particular, there is an unfulfilled need to provide a pneumatic assembly that is compact, easily manufactured, inexpensive, and reliable, yet also is capable of operating at a critical Reynolds number that is higher than otherwise possible.
Accordingly, there is a need for a flow conditioning system for providing a steady fluid stream in one or more flow paths. Further, because such flow systems are sometimes provided in the form of a compact pneumatic assembly having short flow paths that are subject to abrupt transitions, wherein one or more of the fluid streams are likely to become unstable, there exists the practical problem of providing such a flow conditioning system for achieving steady or laminar flow in a compact pneumatic assembly.
Further, there may be a need to sense certain characteristics of the fluid stream at certain points in one or more of the flow paths in a pneumatic assembly. Examples of such sensed characteristics include the pressure, flow rate, and temperature of the fluid, or the presence or absence of a certain component of the fluid stream, such as an analyte or contaminant. Such needs are typically addressed by the integration of one or more sensors in a given flow path. However, most (if not all) of such sensors are designed to be exposed to steady or laminar flow. A sensor positioned in a fluid stream that is subject to unsteady flow is likely to provide false or inaccurate sense data. Some sensors, such as those having a nonlinear response, are accurate only when subjected to a laminar fluid stream. Accordingly, a need exists for integrating a flow conditioning system into a pneumatic assembly for providing steady or laminar flow to a sensor.